1. Field of the Invention
The present invention relates to a drive control system in a series-hybrid vehicle.
2. Description of the Related Art
Recently, to achieve a high fuel efficiency and avert air pollution, it has been proposed that a hybrid power unit is incorporated into a utility vehicle. The hybrid power unit includes an internal combustion engine and an electric motor. Exemplary hybrid power units are a series-hybrid power unit, a parallel-hybrid power unit, a split-hybrid power unit, etc., depending on a connection relation of the internal combustion engine, an electric generator, the electric motor, and wheels.
Japanese Patent No. 3,094,701 discloses a control system in a series-hybrid vehicle which switches a driving mode based on a value of SOC (state of charge) of a battery. In accordance with this control system, if the value of SOC is greater than a first predetermined value, an EV (electric vehicle) mode is selected, and the engine is stopped. If the value of SOC is less than the first predetermined value, an HV (hybrid vehicle) mode is selected, and an engine electric generator is activated. If the value of SOC is less than a second predetermined value, electric power consumed in a motor is limited not to exceed a power generation output of the engine electric generator, and the vehicle drives without consuming the electric power stored in the battery. The second predetermined value is less than the first predetermined value.
However, in the above stated conventional control system, if the value of SOC decreases to a value less than the second predetermined value, a driving power of the motor may possibly decrease rapidly with a great magnitude. In this situation, driving feel may be worsened, or uphill driving or driving on a curve may become difficult suddenly. In the conventional control system, to minimize a rapid decrease in the driving power of the motor, the engine electric generator must increase the power generation output, which would result in an increase in the size of the engine electric generator.